18.3.6 Updating Time Stamps Automatically

You can arrange to put a time stamp in a file, so that it is updated
automatically each time you edit and save the file. The time stamp
must be in the first eight lines of the file, and you should insert it
like this:

Time-stamp: <>

or like this:

Time-stamp: " "

Then add the function time-stamp to the hook
before-save-hook (see Hooks). When you save the file, this
function then automatically updates the time stamp with the current
date and time. You can also use the command M-x time-stamp to
update the time stamp manually. For other customizations, see the
Custom group time-stamp. Note that the time stamp is formatted
according to your locale setting (see Environment).